Demands are increasing in recent years for high sensitivity in the examination of environmental toxins, DNA, cancer cells and disease-causing germs of different types by their immuno-reaction. This demand has spurred intensive development of immunoassay systems. The general method for making an immunoassay is an optical method that labels an antibody (for detection) selectively bound to an antigen for detection, by optically marking it with an optical fluorescent enzyme, detecting the reaction of the bound antibody-antigen as an optical signal from the optical marker to find the type and quantity of the antigen. However, this optical method does not have adequate detection sensitivity and requires a process for washing off the unbound optical marker (BF separation).
A magnetic method for detecting antigen-antibody reactions has been proposed in recent years that utilizes a magnetic nanoparticles and a SQUID magnetic sensor to achieve detection sensitivity surpassing the optical method. This magnetic method detects antibodies magnetically labeled with magnetic nanoparticles (hereafter called magnetic markers), by utilizing a SQUID magnetic sensor possessing extremely high sensitivity.
Methods for detecting magnetic markers are based on:                (1) measuring the susceptibility,        (2) measuring the magnetic relaxation        (3) measuring the residual magnetism (remanence).        
The above methods (1) through (3) are described next.